The speed of a vibration type actuator varies with the amplitude of a vibration wave that is excited by a vibrating member. For this reason, the speed is controlled by varying the amplitude and frequency of an AC voltage to be applied to an electrical-mechanical energy converting element provided at the vibrating member.
The process of controlling speed while varying the frequency of an AC voltage is often used to reduce the effects of temperature-related variations in the resonance frequency of a vibrating member and differences in resonance frequency among individual vibrating members.
However, in the case where a target speed is too high or larger-than-expected load torque or thrust is applied, the target speed may not be reached even when the frequency of the AC voltage reaches the resonance frequency of the vibrating member.
For this reason, the vibration type actuator may decelerate or stop abruptly, and unusual noise may be generated. In some cases, a friction surface of the vibrating member or a movable member may be damaged to degrade the durability performance. The process of allowing sufficient latitude in load conditions and the target speed, the process of measuring a resonance frequency and determining a lowest frequency in advance, and other processes have conventionally been proposed to cope with such phenomena.
There is also proposed the process of measuring a physical phenomenon which varies with the difference between the frequency of an AC voltage as described above and the resonance frequency of a vibrating member and controlling the frequency of the AC voltage so as not to exceed the resonance frequency of the vibrating member.
As one of the methods, PTL 1 proposes the method below.
In the method in PTL 1, a sensor which measures vibration of a vibrating member is provided to detect a phase lag of vibration or vibration amplitude with respect to an AC voltage to be applied to an electrical-mechanical energy converting element. A lower limit frequency for the AC voltage is determined based on the detected data and is stored in memory in advance.
The frequency of the AC voltage is controlled so as not to fall below the lower limit frequency and within a low frequency region.
PTL 2 presents the method below. In the method in PTL 2, an AC voltage for measuring the damping capacitance of a piezoelectric element is superimposed on an AC voltage to be applied to an electrical-mechanical energy converting element of a vibration type actuator to measure the damping capacitance.
A series resonant circuit current obtained by subtracting an AC current flowing into the damping capacitance from an AC current flowing through the piezoelectric element is obtained.
Since the series resonant circuit current flows in proportion to the vibration speed of a vibrating member, the temporal phase difference between the AC voltage applied to the piezoelectric element and the series resonant circuit current is obtained, and the frequency of the AC voltage is controlled such that the phase difference is a predetermined phase (e.g., 0°).
With these operations, the frequency of the AC voltage is made to follow a change in the resonance frequency of a series resonant circuit.
In the method in PTL 1, a resonant condition is detected with a vibration detecting sensor provided at the vibrating member and the phase difference from the applied voltage. Accordingly, in the case where the amplitude of the vibrating member is small, an offset may be superimposed on the phase difference due to a change in the pressurized contact state between the vibrating member and a movable member, and the method suffers from the problem of instability.
It is difficult to cope with a change in the resonance frequency of the vibrating member caused by a temperature change by storing a lower limit frequency in advance during the step of detecting a resonant condition.
In the method in PTL 2, oscillation of the vibrating member is detected by measuring the series resonant circuit current, and a resonant condition is detected by the phase difference between the applied voltage and the vibration. The method has a problem with the accuracy of detecting a phase difference at a frequency away from the resonance frequency when the amplitude of the vibrating member is small.
In the process of detecting a current resonant condition by the vibration sensor, in the case where the frequency of an applied voltage is swept at high speed due to, e.g., an abrupt load variation, a delay in phase detection by the vibration detecting sensor may cause the frequency of the applied voltage to exceed or fall below the resonance frequency of the vibrating member.
The methods in PTL 1 and PTL 2 both require a vibration detecting sensor in order to detect a resonant condition.